How do I apply modifiers and other changes in 2.90?
While Emir's answer is technically correct, it can be tedious to apply multiple modifiers using the Modifier's panel. Note  that, for any selected object, you can also float your cursor anywhere within a given modifier's listing within the Modifier panel and press the Ctrl + a shortcut, and that specific modifier will be applied instantly. Which is nice.
But if you have a lot of modifiers this may still not be very convenient. Thankfully, you can apply all modifiers at once by selecting the object which contains them in Object Mode, and at the top of the 3D View, go to the Object menu, then Convert To, and finally Mesh. Even though the mesh was already a mesh before you began, converting it to mesh applies all its modifiers at once.